SUMMARYAn experimental approach to high-performance zone electrophoresis is given. It is shown that dispersion can be well controlled by the use of narrow-bore tubes made of chekcally and electrically inert materials. The asymmetric concentration distributions that are frequently obtained in free zone electrophoresis are the result of migrational dispersion. This asymmetry only can be suppressed by the application of very small amounts of sample. High-performance separations with UV and conductimetric detection are shown. The time of analysis can be reduced to a few minutes by selecting the appropriate operational conditions. Plate heights smaller than 10 pm can easily be obtained.

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Uremic ultrafiltrates (and normal serum, for comparison) were fractionated by means of gel filtration. The collected fractions were further investigated by combined analytical techniques: "high-performance" liquid chromatography, gas chromatography, mass spectrometry, and isotachophoresis. Ultrafiltrate fractions in the so-called middle molecular mass region (Mr 500-2000) contained a considerable amount of substances of low molecular mass, such as carbohydrates, organic acids, amino acids, and ultraviolet absorbing solutes. Ultraviolet absorbance in the "middle molecular mass region" of the gel chromatogram is mainly due to the presence of these rather low-molecular-mass solutes. Therefore this signal is not a quantitative measure of molecules with a "middle" molecular mass.